Basic Elec ??

[Vagabond]

Ok i made the mod found here -> here

now i have the wires attached and the LED works now my Question is the mod'er here uses a resistor (120-170) on the pos side of the LED ....WHy?? my LED lit with out it?? I dont have it hooked up perm yet waiting to see why i need this resistor...

txs

[Kc2iLq]

The mouse can supply 5V DC. His LED used 2.2v so he supplied it with that. It is possible that your LED can handle 5V (some do).

-Kc2iLq

[John Prophet]

The led needs about 2.2v....the leads on the little ic chip are probably putting out 5volts. The resistor is there as a "drop" or as a type of "voltage divider"...that way the resistor and the led are "dividing" up the voltage so that the led doesnt get too much.

The way you have it setup it may burn out in short order.

What color led did you use? Evidently the blue ones can use more voltage since he said you could get away with a smaller resistor with blue.



So the resistor is there to make it last longer basically.

JP

[John Prophet]

Another method may be to put two leds in a row (instead of the resistor) so that they divide the voltage between each other. Just make sure the leds hook up with pos lead of one led to the neg lead of the other. (youll know, cuz it wont work otherwise)

[Vagabond]

txs everyone for the info I will get a resistor so my LED lasts longer.. also does someone have a good linky for basic electronics... so i could get some more bk on this

txs

[linear1]

I'm going to answer this one differently.

The purpose of the resistor is to limit the current supplied to the LED. It does also drop some voltage, but that's not the most useful way to think about it, and I'll explain why.

Ohm's Law says that the relationship between current and voltage in a resistor is linear, and that the ratio of the voltage/current is the resistance. LED's do not obey Ohm's law.

In fact, the relationship between current and voltage in a diode is exponential--a small change in voltage can produce a huge change in the current that flows. This is why LEDs connected to voltage sources need a current-limiting resistor, and that's why Japala used a resistor. To limit the current to the LED to a safe value (20mA is almost always a safe value).

Let's look at a couple of the suggestions previously given and analyze them:
Kc2iLq>It is possible that your LED can handle 5V (some do).
Tha's correct, but it's an incomplete answer, which is why I'm being so particular about the current. Modern blue LEDs are often rated for 5V, but they will carry a current at that voltage which is well in excess of the 20mA target current, maybe 5-10 times that. While the LED doesn't immediately fail catastrophically, it's forced to dissipate heat (equal to current*voltage) which shortens the life of the diode. Some LEDs are internally current-limited, but it's more common to find those set up to use 12V than 5V.

John Prophet>Another method may be to put two leds in a row (instead of the resistor) so that they divide the voltage between each other.
This is also partly correct. The problem again is that it doesn't limit the current effectively for every possible combination of LEDs, and it forces some assumptions and requires you to account for the cumulative forward voltage.

To illustrate, consider 2 blue LEDS in series with a 5V source. The rated forward voltage of each LED at 20 mA is somewhere around 3.6V, so this arrangement has a cumulative forward voltage of 7.2V, and the source will not drive the LEDs to the target 20 mA current. They may not light up at all, but they certainly won't be at full brightness. Two red LEDs would probably be slightly overdriven in this circuit, since you're driving them above the rated current.

The advantages of resistors are that they're cheap, and you can easily calculate the size you need to limit current to a safe value if you know your LED's specs.

[John Prophet]

http://science-ebooks.com/electronic...lectronics.htm

[John Prophet]

Thank you linear...lol..but I was going with the thought that he probably didnt want or need an in depth analysis of ohms law, lol.

Its much easier for people to understand voltage..besides since voltage,resistance and current are irrevocably tied together..speaking of one is pretty much the same as speaking of another.

And to explain my simplified thinking with the two leds in line....I figured two in line would be better than one by itself, lol...no matter WHAT the resistance of the leds (or resistors)..if they are equal and in series like that..and across a 5v line..they will have 2.5 volts across them..very close to his stated 2.2.

[Vagabond]

txs guys good reading and good link ill do some more reading...

[linear1]

JP>since voltage,resistance and current are irrevocably tied together
That's true only for resistors, though.

JP>resistance of the leds
LEDs don't have resistance.

The answer to the original question "why do I need a resistor" is not adequately answered without mentioning current. You need a resistor to limit the current the LED carries. I don't really consider the other answers wrong, they are just incomplete.